Pumps used in SFC Supercritical Fluid Chromatography control the mass-flow of each component of the mobile phase, and therefore control the composition of the mobile phase through the column. Pumping compressible fluids, such as carbon dioxide (CO2), at high pressures in SFC systems while accurately controlling the flow, is more difficult than that for a liquid chromatography system. SFC systems use two pumps to deliver two different source streams into a single mobile phase flow stream. Each pump on each flow stream adds pressure, flow ripples, and variances that cause baseline noise. The two pumps also operate at different frequencies, different flow rates, and require separate compressibility compensations, further adding to the complexity of flow operations.
Pumps used for supercritical fluid chromatography typically require an extended compressibility compensation range plus a dynamically variable compressibility compensation range to accurately deliver a desired flow rate and fluid composition. When a compressible fluid is mixed with an incompressible fluid, the viscosity of the mixture depends on the mole fraction of the modifier, or incompressible fluid, in the compressible fluid. In combi-chem analysis and purification by SFC, the concentration of modifier can be varied from 2.5 to 55% over a few minutes. This can result in a major change in the viscosity of the fluid and in the pressure drop across a chromatographic column that can change over an order of magnitude from approximately 9 bar to greater than 250 bar.
Normally, an unmodified high performance liquid chromatography (HPLC) pump would deliver an unknown and varying amount of a compressible fluid under such conditions. As the column head pressure increases during the gradient, a larger percentage of each pump stroke would be used up compressing the fluid instead of delivering flow. With an uncompensated pump, the delivery rate becomes a smaller fraction of the flow setpoint. When a second pump is added to a system to deliver an incompressible fluid under high pressure, its delivery rate is unaffected by the increasing pressure. Subsequently the two pumps deliver inaccurate flow and composition to the mobile phase. As the pressure in the system rises, the total flow drops below its setpoint, but the concentration of the modifier increases beyond the modifier setpoint. The temperature of the compressible fluid in the pump head must be controlled to prevent the delivered mass flow from changing even further.
When compressed, a pumping fluid heats up and attempts to expand. For highly compressible carbon dioxide at outlet pump pressures above 200 bar, temperature rise of more than ten degrees centigrade are possible within the fluid. The rapid compression of the pumping fluid causes the fluid to heat up and expand and the density to decrease. When heat is transferred to the pump body, the pumped fluid cools and the fluid density increases.
Reciprocating pumps are typically used in HPLC and SFC. These pumps are more accurate than syringe pumps and can deliver essentially an infinite volume before refilling. A reciprocating pump has an inlet and outlet check valve. During a fill stroke, the outlet check valve closes, isolating the pump from the high pressure in the downstream column (Pcol). The pressure from a filling cylinder of source fluid, such as compressed CO2, (Pcyl) forces open the inlet check valve and fills the pump chamber. After the pump is filled at Pcyl, the piston reverses direction, compressing the fluid in the pump until Ppump greater than Pcyl, which closes the inlet valve. On the compression stroke, the piston moves rapidly until Ppump greater than Pcol at which point the outlet valve opens and the fluid moves downstream of the pump and into the column. The piston slows down to the delivery speed when enough extra fluid has been pushed into the column to compensate for lack of flow during the fill stroke. The distance the piston must travel just to compress the fluid to Pcol is calculated based on the known volume of the components and a characteristic of the fluid being pumped, called the compressibility factor Z. With the correct Z, a pump can be controlled to nearly eliminate flow or pressure ripple.
Without a correct Z, the pump either under- or over-compresses the fluid causing characteristic ripples in flow and pressure. Either under- or over-compression results in periodic variation in both pressure and flow with the characteristic frequency of the pump (ml/min divided by pump stroke volume in ml). The result is noisy baselines and irreproducibility. To compensate for this, the more expensive and better liquid chromatography pumps have compressibility adjustments to account for differences in fluid characteristics.
High-pressure SFC pumps have an extended compressibility range and the ability to dynamically change the compression compensation. While these pumps are used as flow sources and the pressure and temperature of the delivered fluid may be measured. The pumps can change the length of compression to account for changes in compressibility with pressure and temperature. Methods in the prior art calculate ideal compressibility based on measured temperature and pressure using a sophisticated equation of state. The method then uses dithering around the setpoint to see if a superior empirical value can be found. This approach is described in U.S. Pat. No. 5,108,264, Method and Apparatus for Real Time Compensation of Fluid Compressibility in High Pressure Reciprocating Pumps, and U.S. Pat. No. 4,883,409, Pumping Apparatus for Delivering Liquid at High Pressure. Other prior art methods move the pump head until the pressure in the refilling cylinder is nearly the same as the pressure in the delivering pump head. One method in U.S. Pat. No. 5,108,264 Method and Apparatus for Real Time Compensation of Fluid Compressibility in High Pressure Reciprocating Pumps, adjusts the pumping speed of a reciprocating pump by delivering the pumping fluid at high pressure and desired flow rate to overcome flow fluctuations. These are completely empirical forms of compressibility compensation. The prior art methods require control of the fluid temperature and are somewhat limited since they do not completely compensate for the compressibility. The compensation stops several hundred psi from the column inlet pressure.
The compressibility of the pumping fluid directly effects volumetric flow rate and mass flow rate. These effects are much more noticeable when using compressible fluids such as carbon dioxide in SFC rather than fluids in liquid chromatography. The assumption of a constant compressibility leads to optimal minimization of fluid fluctuation at only one point of the pressure/temperature characteristic, but at other pressures and temperatures, flow fluctuations occur in the system.
If the flow rate should be kept as constant as possible through the separation column. If the flow rate fluctuates, variations in the retention time of the injected sample would occur such that the areas of the chromatographic peaks produced by a detector connected to the outlet of the column would vary. Since the peak areas are representative for the concentration of the chromatographically separated sample substance, fluctuations in the flow rate would impair the accuracy and the reproducibility of quantitative measurements. At high pressures, compressibility of solvents is very noticeable and failure to account for compressibility causes technical errors in analyses and separation in SFC.
The invention relates to a method and apparatus for converting a pump for use in a combined flow stream containing a mixture of highly compressed gas, compressible liquid or supercritical fluid; and a relatively incompressible liquid. In particular, the invention pertains to converting a pump with a constant compressibility compensation for use in gradient elution supercritical fluid chromatography (SFC).
The problem addressed by the present invention is how to accomplish accurate composition of a flow stream and steady flow rates while avoiding variable compression compensation adjustments to a flow pump that is delivering a highly compressible fluid, such as carbon dioxide, to a supercritical fluid chromatography class of system. In the present invention, unmodified HPLC pumps can to deliver reproducible flow conditions of a pure fluid under isocratic conditions to an SFC system, despite having limited compressibility compensation ranges and no ability to dynamically compensate for compressibility changes. The pump delivers a compressible fluid against a back-pressure regulator installed just downstream of the pump""s outlet, thereby holding a pressure force against the pump and delivering a steady, high pressure flow stream to a separation column. Pressures can reach upwards of approximately 600 bar in the flow stream. The result of the invention is controllable flow of a compressible fluid delivered downstream of the regulator to the mobile phase and into the SFC column without performing dynamic compression compensation or other types of compensations for leaks and ripples in flow on the pump.
In the present invention, calculating variable compressibility compensation is avoided when pumping compressible fluids in a supercritical fluid chromatography system. Normally a pump has to change the nature of how it moves to compress fluids, and once the fluid is compressed, then the pump delivers the fluid. The hardware and methods for performing precise compressibility in laboratory-scale pumps are difficult and expensive. By using a relatively inexpensive pump without precise mechanics, outlet pressure is controlled with the current invention, thereby significant reducing capital and operating laboratory costs. Pressure downstream of the back pressure regulator 12 may vary according to the dynamics of the SFC system. However, the pressure delivery out of the pump 10 is a constant delivery to the system.
A back pressure regulator is mounted just downstream of the pump in a supercritical chromatography system and controls the pump""s outlet pressure above the inlet pressure, while maintenance pressure drop across the pump constant. The back pressure regulator could be mechanical, or electromechanical, or thermally controlled. Any of the types of back pressure regulators work with the preferred embodiment. The density of the fluid in the pump varies over a carefully controlled range during refill and delivery. If the inlet pressure is relatively high the fluid is less compressible. If the temperature of the fluid is then maintained constant, sub-ambient level, the fluid is still less compressible and there is no change in the compressibility during a run. In an alternative exemplary embodiment, a separate system could pre-pressurize the fluid entering the pump to an elevated pressure.
In an alternative exemplary embodiment, a separate system could pre-pressurize the fluid entering the pump to an elevated pressure. A back pressure regulator is mounted just downstream of the pump and controls the pump""s outlet pressure above the inlet pressure, while maintaining the pressure drop across the pump constant. The density of the fluid in the pump varies over a carefully controlled range during refill and delivery. If the inlet pressure is relatively high the fluid is less compressible. If the temperature of the fluid is then maintained constant, sub-ambient level, the fluid is still less compressible and there is no change in the compressibility during a run.